1. Field of the Invention
The present invention relates to a drive apparatus for a self light-emitting display.
2. Description of Related Art
To realize a gradation display on a plasma display panel as a self light-emitting display unit, there is a known method which divides the display period of one frame (field) into N subframes (subfields) to permit light emission only for the time corresponding to the weight on each bit position of N-bit display data (so-called subfield method).
When pixel data consists of eight bits, for example, the display period of one frame is divided into eight subframes SF8, SF7, SF6, . . . , and SF1 in the order of a heavier weight to a lighter one. At this time, light emissions of 128 pulses, 64 pulses, 32 pulses, 16 pulses, 8 pulses, 4 pulses, 2 pulses and 1 pulse are carried out in the respective subframes SF8 to SF1. The light emissions in those eight subframes provide 256-gradation display.
Because of the fixed display order for the subframes SF8-SF1, this gradation display scheme however has such a problem that a moire-like false outline which looks like a gradation-lost image is observed near the area on a flat image where the gradation level crosses the boundary of 2.sup.n gradation levels, such as 128 or 64, which significantly degrades the display quality.
A gradation display scheme which solves this problem has been proposed in, for example, Japanese Patent Kokai (laying open) No. Hei 7-271325. This gradation display scheme suppresses a pseudo outline by equally dividing a subframe with a heavy weight into a plurality of subframes, separating them so as to prepare a plurality of light emission patterns which have the equal light emission time (the equal number of light emissions) with different light emission orders of the subframes, and changing the light emission pattern from one to another pixel by pixel (pseudo outline compensation data conversion).
This gradation display scheme however results in an increased number of subframes in one frame period. If the number of bits of pixel data is increased to improve the image quality, the number of subframes in one frame period is increased more.
The increase in the number of subframes in one frame period increases the addressing period for lighting a plasma display panel for light emission. This relatively shortens the sustain period as the light emission period, reducing the maximum luminescent.
In this respect, a dithering process which reduces the number of bits (the number of subframes) of pixel data and effects pseudo intermediate tone display is performed.
The dithering process expresses a single intermediate display level with a plurality of adjacent pixels. In the case where 8-bit equivalent gradation display is demonstrated using the upper six bits of pixel data in 8-bit pixel data, for example, four dither coefficients different from one another are respectively assigned to and added to pixel data corresponding to the individual pixels in each set of four pixels adjoining right and left and up and down are added to pixel data.
FIG. 1 is a diagram illustrating the correlation between dither coefficients "a" to "d" to be added to pixel data by this dithering process, and the individual pixels.
For example, the dither coefficient "a" is added to pixel data corresponding to the pixel at the first row and the first column, the dither coefficient "b" is added to pixel data corresponding to the pixel at the first row and the second column, the dither coefficient "c" is added to pixel data corresponding to the pixel at the second row and the first column, and the dither coefficient "d" is added to pixel data corresponding to the pixel at the second row and the second column.
Those dither coefficients "a" to "d" are respectively added to the pixel data of the individual pixels in each set of four pixels adjoining right and left and up and down are added to pixel data, as indicated by the broken lines in FIG. 1.
Then, the upper six bits of the dither-coefficients added pixel data are extracted to be used as a drive signal for the display panel.
This dithering process generates a combination of four different intermediate display levels with four pixels, thus ensuring four times the 6-bit gradation display levels or 8-bit equivalent intermediate tone display.
If the dither pattern consisting of the dither coefficients "a"-"d" is always added to the individual pixels as indicated by the broken lines in FIG. 1, however, noise originating from this dither pattern may appear, thus degrading the image quality.